1. Field of the Invention
The present invention relates to a method for effectively recovering nickel as a useful metal from a sludge by increasing the proportion of a nickel constituent contained in the sludge at the time of precipitating as the sludge the nickel dissolved in the ferric chloride waste liquid containing nickel and other heavy metals such as the etching waste liquid during reproduction of the ferric chloride waste liquid.
Generally, the ferric chloride waste liquid is used broadly as an etching liquid for precisely processing a metal plate made of stainless steel, nickel alloy and so on. As the etching liquid is used repeatedly for the corrosive treatment, it is apt to accumulate heavy metal ions such as nickel ion and the like therein as well as tervalent ions are reduced to bivalent ones. Meantime the etching liquid loses its corrosive effect and finally becomes the waste liquid. Since many pollution-related problems arise when this waste liquid is dumpted much after having been merely neutralized by alkali, it is requested to apply the reproductive and recycle process to the waste liquid.
2. Description of the Prior Art
As a method for reproducing the aforementioned waste liquid it might be proposed to blow the chlorine gas into the waste liquid directly so as to oxidize the bivalent iron ions to the tervalent ones. Practically, however, a corrosive effect of the reproduced process liquid doesn't recover sufficiently by this processing method. The reason is assumed that the heavy metal ions such as the nickel ion dissolved from the metallic plate into the waste liquid by the etching process tend to obstruct the oxidizing reaction.
Then, such a reproduction processing method for recovering the corrosive effect sufficiently is proposed by the Japanese Patent Publication No. Sho 61-44814. According to this reproduction processing method, lumps of metallic iron are added to the waste liquid and then heated and stirred so as to precipitate heavy metals. After the precipitated metals have been removed as a sludge, the chlorine gas is blown into the waste liquid so as to reproduce the liquid to the ferric chloride liquid.
But, the above-mentioned reproduction processing method requires an excessive addition of the metallic iron for precipitating the heavy metals. The unreacted portion of this excessive metallic iron is apt to be contained much in the sludge produced. That is, since most of the metallic constituents in the sludge is occupied by the iron constituent, the proportion of useful metals such as the nickel contained therein is not very high. For example, in the case that the reproducing process is performed by addition of the metallic iron to the ferric chloride waste liquid produced by the etching for stainless steel, the iron constituent of the metal constituents in the sludge occupies more than 90% and the nickel constituent is less than 10%. As a result, since it is impossible to effectively recover the useful metals such as the nickel from the obtained sludge, disadvantageously an economical value of the sludge is low.
On one hand, in order to reproduce the ferric chloride waste liquid effectively, formerly the inventor of the present invention proposed such a method as to increase a processing reaction speed by adding metallic copper together with the metallic iron (refer to the Japanese Patent Layed Open Publication No. Hei 2-229724). But, there are several problems associated with this method, although the processing reaction speed is increased, that the metallic copper increases the reproduction cost while similarly to the prior art, since the sludge contains the excessive unreacted metallic iron much, the proportion of such useful metals contained in the sludge is low. Further, since the sludge contains also much added metallic copper, the purity of the precipitated metallic nickel is low and the economical value of the sludge is still low.